从像素坐标到opengl坐标

Question

正如我在opengl中看到的坐标介于-1和1之间，我想使用opengl渲染对象，我有第一个位置在哪里以像素坐标渲染对象，但我不知道如何将像素坐标转换为opengl坐标。我正在使用java的android应用程序。

public class FirstOpenGLProjectRenderer实现GLSurfaceView.Renderer {

private static final String A_POSITION = "a_Position";
private static final String A_COLOR = "a_Color";
private static final int POSITION_COMPONENT_COUNT = 4;
private static final int COLOR_COMPONENT_COUNT = 3;
private static final int BYTES_PER_FLOAT = 4;
private static final int STRIDE =
        (POSITION_COMPONENT_COUNT + COLOR_COMPONENT_COUNT) * BYTES_PER_FLOAT;

private final FloatBuffer vertexData;
private final Context context;

private int program;
private int aPositionLocation;
private int aColorLocation;

private static final String U_MATRIX = "u_Matrix";
private final float[] projectionMatrix = new float[16];
private int uMatrixLocation;
private final float[] modelMatrix = new float[16];



public FirstOpenGLProjectRenderer(Context context) {
    this.context = context;


 //
    // Vertex data is stored in the following manner:
    //
    // The first two numbers are part of the position: X, Y
    // The next three numbers are part of the color: R, G, B
    //
    float[] tableVerticesWithTriangles = {
            // Order of coordinates: X, Y, R, G, B


            // Triangle Fan
            0f, 0f, 0f, 1.5f, 1f, 1f, 1f,
            -0.5f, -0.8f, 0f, 1f, 0.7f, 0.7f, 0.7f,
            0.5f, -0.8f, 0f, 1f, 0.7f, 0.7f, 0.7f,
            0.5f, 0.8f, 0f, 2f, 0.7f, 0.7f, 0.7f,
            -0.5f, 0.8f, 0f, 2f, 0.7f, 0.7f, 0.7f,
            -0.5f, -0.8f, 0f, 1f, 0.7f, 0.7f, 0.7f,
        // Line 1
            -0.5f, 0f, 0f, 1.5f, 1f, 0f, 0f,
            0.5f, 0f, 0f, 1.5f, 1f, 0f, 0f,
        // Mallets
            0f, -0.4f, 0f, 1.25f, 0f, 0f, 1f,
            0f, 0.4f, 0f, 1.75f, 1f, 0f, 0f

    };

    vertexData = ByteBuffer
            .allocateDirect(tableVerticesWithTriangles.length * BYTES_PER_FLOAT)
            .order(ByteOrder.nativeOrder()).asFloatBuffer();

    vertexData.put(tableVerticesWithTriangles);
}

@Override
public void onSurfaceCreated(GL10 glUnused, EGLConfig config) {
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

    String vertexShaderSource = TextResourceReader
            .readTextFileFromResource(context, R.raw.imple_vertex_shader);
    String fragmentShaderSource = TextResourceReader
            .readTextFileFromResource(context, R.raw.simple_fragment_shader);

    int vertexShader = ShaderHelper.compileVertexShader(vertexShaderSource);
    int fragmentShader = ShaderHelper
            .compileFragmentShader(fragmentShaderSource);

    program = ShaderHelper.linkProgram(vertexShader, fragmentShader);

    if (LoggerConfig.ON) {
        ShaderHelper.validateProgram(program);
    }

    glUseProgram(program);

    uMatrixLocation = glGetUniformLocation(program, U_MATRIX);

    aPositionLocation = glGetAttribLocation(program, A_POSITION);
    aColorLocation = glGetAttribLocation(program, A_COLOR);

    // Bind our data, specified by the variable vertexData, to the vertex
    // attribute at location A_POSITION_LOCATION.
    vertexData.position(0);
    glVertexAttribPointer(aPositionLocation, POSITION_COMPONENT_COUNT, GL_FLOAT,
            false, STRIDE, vertexData);

    glEnableVertexAttribArray(aPositionLocation);

    // Bind our data, specified by the variable vertexData, to the vertex
    // attribute at location A_COLOR_LOCATION.
    vertexData.position(POSITION_COMPONENT_COUNT);
    glVertexAttribPointer(aColorLocation, COLOR_COMPONENT_COUNT, GL_FLOAT,
            false, STRIDE, vertexData);

    glEnableVertexAttribArray(aColorLocation);

}


@Override
public void onSurfaceChanged(GL10 glUnused, int width, int height) {
    // Set the OpenGL viewport to fill the entire surface.
    glViewport(0, 0, width, height);

    MatrixHelper.perspectiveM(projectionMatrix, 45, (float) width
            / (float) height, 1f, 10f);



    setIdentityM(modelMatrix, 0);

    translateM(modelMatrix, 0, 0f, 0f, -2.5f);
    rotateM(modelMatrix, 0, -60f, 1f, 0f, 0f);

    final float[] temp = new float[16];
    multiplyMM(temp, 0, projectionMatrix, 0, modelMatrix, 0);
    System.arraycopy(temp, 0, projectionMatrix, 0, temp.length);

/ }

@Override
public void onDrawFrame(GL10 glUnused) {
    // Clear the rendering surface.
    glClear(GL_COLOR_BUFFER_BIT);

    // Assign the matrix
    glUniformMatrix4fv(uMatrixLocation, 1, false, projectionMatrix, 0);

    // Draw the table.
    glDrawArrays(GL_TRIANGLE_FAN, 0, 6);

    // Draw the center dividing line.
    glDrawArrays(GL_LINES, 6, 2);

    // Draw the first mallet.
    glDrawArrays(GL_POINTS, 8, 1);

    // Draw the second mallet.
    glDrawArrays(GL_POINTS, 9, 1);
}

}

Answer 1

首先，如果您只是想将2D图元绘制到屏幕上，您希望不应用任何3D模型，视图或投影矩阵（即使用仅通过未改变的位置传递的着色器，或将这些矩阵设置为标识） 。然后你基本上可以直接指定要在剪辑空间中绘制的东西的坐标。剪辑空间实际上是一个projective space和剪辑空间坐标实际上是四维homogeneous coordinates，但你现在不必担心这一点。只要将第四个坐标w设置为1，剪辑空间坐标将直接对应于标准化设备坐标，这是您已经熟悉的-1到1坐标系。

那么，除此之外，我们如何从像素坐标到标准化设备坐标？那么，我们需要做的是将[0..w）和[0..h]范围的x和y像素索引映射到[-1,1]范围内的位置（其中w和h是数字视口/屏幕的像素分别在x和y方向上）。这里要注意的一件重要事情是OpenGL中的像素位置对应于采样网格的单元的中心。这意味着像素（0,0）不会落在左下角，而是偏移“1/2像素”。如果你原谅我的ASCII艺术：

    |     |     |     |   
    +-----+-----+-----+---
    |  o  |  o  |  o  |   
    +-----+-----+-----+---
    |  o  |  o  |  o  |   
    +-----+-----+-----+---
    |  X  |  o  |  o  |   
    +-----+-----+-----+---
(-1, -1)

像素位置之间的距离在x中为1 / w，在y方向上为1 / h。因此，标准化设备坐标中像素（x，y）的位置是：

x_ndc = 2.0f * (x + 0.5f) / w - 1.0f;
y_ndc = 2.0f * (y + 0.5f) / h - 1.0f;

并且在该点放置顶点的​​齐次坐标将是(x_ndc, y_ndc, -1.0f, 1.0f)。请注意，z坐标可以是您想要放置2D元素的任何深度（-1对应于近平面，+对应于远平面）。